Carrier-current phase-angle relaying system



30, 1951 H. w. LENSNER ET AL 2,

CARRIER-CURRENT PHASE-ANGLE RELAYING SYSTEM Filed June 30, 1947 WITNESSE Herberf W Lensner,Sh/'r/e L. Go/a'sborou h 6% a y 9 4% and Poberf C2 Cheek.

ATTOR NEY Patented Jan. 30, 1951 UNITED STATES PATENT OFFICE CARRIER- CURRENT PHASE-ANGLE RELAYING SYSTEM Application June 30, 1947, Serial N0. 758,200

Claims.

Our present invention is an improvement on protective relaying systems of the type in which carrier current is utilized to effect a direct comparison of the phase-angles of the currents at the two ends, or at a plurality of terminals, of the protected line-section of an alternating-current transmission system. More specifically, our invention is an improvement in the type of carrier-current phase-angle relaying system which is shown in the Mehring Patent 2,408,868, granted October 8, 1946, which is characterized by a pair of alternately fired gas-filled tubes, which produce square-topped voltage-waves during alternate half-cycles of the line-current at the relaying' station, one of said tubes being utilized to transmit restraining impulses to the opposite end of the protected line-section, by the aid of carrier, and the other gas-tube being utilized to apply operating impulses to the phase-angle-responsive relay. The Mehring system utilized a single fault-detector for setting the carrier-controlling gas-tube into operation, and the other gas-tube was biased so that it would not respond quite as sensitively as the fault-detector, in response to fault-currents.

The Mehring patent also utilized a line-current-responsive network for deriving a singlephase alternating-current quantity which is responsive to a plurality of different line-fault conditions, on a 3-phase line, as broadly covered by a the Harder Patent 2,183,646, granted December 19, 1939. The Mehring patent was a more direct out growth of a phase-angle-responsive relayingsystem of the type shown in the Bostwick Patent 2,275,941, granted March 10, 1942, which effected a phase-angle comparison without utilizing a pair of alternately triggered gas-tubes for producing the two series of constant-magnitude,

square-topped voltage-impulses, onalternate line-current half-cycles, for respectively producing the operating and restraining impulses. The Bostwick system utilized two fault-detectors of differing sensitivities; the more sensitive, or lowset, fault-detector being utilized to supervise the transmission of carrier on alternate line-current half-cycles, while the less sensitive, or high-set, fault-detector was utilized to supervise the phase-angle relay. The Bostwick system required carrier-current of a difierent frequency for each line-terminal, and it also involved a difficult problem of correlating the settings of a plurality of different kinds of current-responsive and current-limiting devices.

Our present invention is more particularlvrelated to the Mehring system, which was one of the most successful forms of phase-angle-responsive carrier-current relaying, prior to our invention. The outstanding feature which contributed most largely to the success of the Mehring systerm was the use of the two alternately firing gastubes which were controlled by a derived singlephase current which was responsive to all kinds of faults, on any of the phases of a 3-phase trans-' mission-system. We avoid, however, two disadvantages of the Mehring system which, for a' while, could not be avoided, for lack of any obvious way to do so, without sacrificing the one most important or essential feature which has just been described.

In the Mehring system, it was necessary .to' bias the second gas-tube, so that its firing-pointj would be higher than, say of the pick-up value of the fault-detector. This had the disadvantage of sometimes transmitting carrier continuously, as a result of the firing of the first gas-tube, under fault-conditions which were not sufiiciently severe to initiate the firing of the second gas-tube on alternate half-cycles of they line-current, thus producing a blind-spot in the relay, which not only prevented the tripping of the relay at the terminal where such a condition arose, but also, because of the continuous transmission of a carrier-signal, which was utilized for blocking, preventing a relay-operation at an other terminal of the line-section in question. It also posed a difficult problem of main-- taining the proper correlation between the pickup value of a mechanical relay, which is aifected only by the severity of the derived line-current,

and the pick-up value of a gas-tube, which is dependent also upon any variation in the directcurrent supply-voltage for the tube. The quicker action of the tube also involved a difliculty, in correlating the pick-up value of the second gastube with the pick-up value of the mechanical fault-detector relay in the Mehring system, because of the sensitivity of the tube to very-shortduration transients in either the alternatingcurrent source or th direct-current system to which the tube is connected.

An object of our invention i to provide a system of the general class described, in which it is impossible to continuously transmit carrier current in response to a fault.

A further object of our invention is to provide a system of the class described, in which no dependence is placed upon the firing-point of one of the alternately triggering gas-tubes, for the trip-setting.

A more specific object of our invention is to provide a relay system in which the firing-points of both of the gas-tubes are lower than the pickup value of a first mechanical fault-detector relay, which has a low setting, while using a second mechanical fault-detector relay, having a higher setting, to prevent the effective operation of the phase-angle relay unless this second fault-detector relay also reponds. This correlation is needed, in order to make sure that a succession of restraining impulses shall always be applied to the relay, before, or at least simultaneously with, the application of a succession of operating-impulses to the relay.

With the foregoing and other objects in View, our invention consists in the systems, circuits, combinations, elements and methods of design and operation, which are hereinafter described and claimed, and illustrated in the accompanying drawing, the single figure of which is a much simplified diagrammatic View of circuits and apparatus illustrative of our invention in a preferred form of embodiment.

In the drawing, we have illustrated our invention as applied to one terminal of a line-section I of a 3-phase transmission-line, which is connected to a S-phase station-bus 2 by means of a circuit-breaker 3 having a trip-coil TC and an auxiliary maize-contact 3a. We have shown only one terminal of the protected line-section l, with the understandingthat the other terminal is, or may be, a duplicate of the terminal equipment which is illustrated.

We utilize a bank of line-current transformers 4, which respond to the 3-phase line-current in the protected line-section i, and supply this current to any suitable network or filter, which is marked HCE, for deriving a single-phase alternating-current voltage which is applied to the primary winding of a saturable transformer ST. Any suitable network, such as HCB, may be utilized, for deriving a single-phase relaying current or voltage which is reasonably uniformly responsive to a plurality of kinds and severities of faults on whatever line-phase a fault may occur. The secondary winding 6 of the saturable transformer ST is shunted by a voltage limiting gas-filled tube GTI, as described in the Harder patent.

The secondary winding 6 is utilized to control two mechanical fault-detector relays FD! and FDZ having diverse sensitivity-settings, and also to control two alternately triggering gas-filled tubes V! and V2. The operating-coils of the two fault-detector relays FD! and FDZ are shown as being energized in series with each other, from the output-terminals of a rectifier-bridge RB, which is supplied with energy from the secondary winding 6 of the saturating transformer ST. The gas-filled tubes Vi and V2 are controlled from the aforesaid secondary winding 6 by having the tube-grids GI and G2 energized, through resistors R! and R2, from two secondary windings H and i2 of an input-transformer IT, the primary of which is energized from the secondary winding of the saturated transformer ST. The other two terminals of the secondary windings I I and 12 of the input-transformer 1T are shown as being connected together, in a circuit 54 Which is connected to an intermediate tapped-point of a cathode-circuit biasing-resistor R3, which is in the cathode-circuits of the gas-tubes Vi and V2.

As shown in the drawing, one terminal of the cathode-circuit biasing-resistor R3 is connected to the negative bus of a direct-current voltage-source for the tubes, while the other terminal of said resistor is connected to a conductor I5, which is utilized for several purposes. The circuit !5 is utilized to energize one terminal of a cathode-circuit loading-resistor R6, the other terminal of which is connected to the cathodeterminal 29 of the first gas-tube VI. The circuit i5 is also utilized to energize one terminal of another cathode-circuit loading-resistor R5, the

other terminal of which is connected to the cathode-circuit 22 of the second gas-tube V2.

The two grid-terminals of these tubes Vi and V2 are connected to their respective cathodecircuits 2i and 22 through capacitors Cl and C2, as is known in the art. The two plate-circuits PI and P2 of these two tubes are connected together through a capacitor C3 which assists in the firing-transfer, as explained in the Mehring patent. The two cathode-circuit loading-resistors RA and R5 are respectively shunted by capacitors C4 and C5, which also assist in the firing-transfer, as set forth in the lVlehring patent. The circuit I l, which is connected to an intermediate tap of the cathode-circuit biasingresistor R3, is also connected to the circuits of the screen grids SGI and SGZ of the respective gas-tubes Vi and V2. The two plate or anodecircuits PI and P2 of the two gas-tubes are connected, respectively, through resistors R5 and El, to a common conductor 24, which is in turn connected to the positive bus through a resistor R8.

The two alternately firing gas-tubes Vi and V2 are supervised by the low-set fault-detector FDH, by having the positive tube-circuit 24 connected to the negative terminal through the normally closed back-contact 25 of the low-set fault-detector FDI.

The circuits of the several relays which are utilized in our invention are arranged, as far as practicable, after the manner of a schematic diagram or across-the-line diagram. In each case, the main or operating coil of the relay is given a letter-designation or legend, and the same letter-designation or legend is applied to all of the contacts of that relay. The relays and switches are invariably shown in their open or deenergized positions. Arrows or dotted lines are used, to symbolically indicate how the various parts of each relay are connected together.

The back-contact 25 of the low-set faultdetector FDE is also utilized to control the enerization of an auxiliary relay K, by having the operating-coil K of this auxiliary relay connected, through a resistor R9, across the FD! back-contact. The auxiliary relay K has a single back-contact K, which is connected in series with a telemetering key TM, which is indicated byway of illustration of carrier-current control for purposes other than relaying. The auxiliaryrelay back-contact K thus acts substantially like a back-contact placed upon the low-set fault-- detector FDI, said auxiliary relay K being utilized for supplying this contact in order to avoid overloading the low-energy fault-detector FDI. The cathode-circuits 2i and 22 of the respective gas-filled tubes Vi and V2 are utilized as sources of two alternating series of square-topped positive-voltage impulses, for two difierent purposes. These positive-voltage impulses are the voltagedrops through the respective cathode-circuit loading-resistors Rd and R5, which have voltagedrops therein when their respective tubes VI and V2 are firing.

The cathode-circuit 2! of the first gas tube VI is utilized to energize the plate-circuit P3 of aj carrier-current master-oscillator tube through a radio-frequency choke-coil RFC-l. During fault-free conditions of the transmissionsystem, the gas-tubes VI and V2 are not firing, because of the short-circuiting back-contact FDI and at such times the carrier-current oscillator OSC may be energized through the telemetering key TVI, and the back-contact K, which connect the circuit 2! to the positive bus through a resistor RI9.

The transmitter-oscillator OSC has its screengrid SG3 connected to the plate-supply circuit 2| of said oscillator. The cathode-circuit of the oscillator OSC is the previously-mentioned circuit or conductor 15. The oscillator has a gridcircuit G3, which is connected to the cathodecircuit I5 through a grid-resistor R-l l.

The plate-circuit P3 of the oscillator CS is connected, through a blocking-capacitor 301, to an intermediate terminal 21 of a tuned carrierfrequency circuit, comprising the conductor 21, a capacitor C6, a conductor 28, a capacitor C1, the cathode-circuit l5, a capacitor C8, the grid-circuit G3, and a variometer Ll, the other terminal of which is connected to the starting-point 21 of the tuned circuit.

The conductors 28 and G3 of this tuned circuit are respectively utilized to apply radioor carrier-frequency control-voltages, through blocking-capacitors B02 and B03 respectively, to the grids of two amplifier-tubes Al and A2. The cathodes of the amplifiers A! and A2 are connected to the cathode-circuit l5 of the oscillator 080. The grids of the amplifier tubes Al and A2 are connected, through grid-resistors GR! and GRZ, to the negative bus so as to apply a negative bias equal to the drop across the oathode-circuit biasing-resistor R3. The two plates of the amplifiers Al and A2 are connected to the primary-winding terminals of a radio-frequency output-transformer OT. The primary winding of said output-transformer OT has a midpoint tap 3B which is connected to the positive supplyterminal and also to the screen-grids of the two amplifiers Al and A2.

The radio-frequency output-transformer OT has a secondary winding 3|, having one of its terminals grounded, and having two taps 32 and 33. The output-transformer secondary-tap 32 is connected to phase-C of the line I, through a variometer L2, a conductor 34, and a couplercapacitor CC. The conductor 34 is also grounded through a grounding-coil GC. The secondary tap 33 of the radio-frequency output-transformer OT is utilized to energize the primary winding of a receiver-coupling transformer RCT, through a tuning-capacitor T-C. The primary winding of the receiver-coupling transformer RCT is also preferably protected by a shunt-connected voltage-limiting gas-filled tube GT2.

The receiver-coupling transformer RCT has a secondary winding 36 which is part of a tuned receiving-circuit, comprising said secondary winding 35, the grid-conductor G4 of a receiver tube REC, a tuning-capacitor C9, the circuit or conductor is and thence back to the secondary winding 36.

The receiver-tube REC is a saturating-tube which carries a plate-cathode current which is of a substantially constant magnitude, whenever the tube is conducting at all, substantially regardless of the voltage applied to the grid-circuit G4, provided that this grid-voltage is high enough to cause plate-current to flow.

The receiver-tube REC has its cathode-circuit 87 energized from a tapped point of a potentiom eter PO 1 which is connected between the circuit [5 and the positive bus The receiver-tube REC has a plate circuit P4, which is energized from the positive supply-terminal through a radio-frequency choke-coil RFC-2. In the rawing illustrative of our invention, we have symbolically shown the telemetering relays 38 in series with the plate-circuit P4, although it should be understood that, in general, the telemetering relays 38 would be utilized only at one line-terminal, while the telemetering key TM would be utilized at the other line-terminal. The

receiver-tube REC also has a screen-grid circuit SCH! which is connected to the positive terminal The receiver-tube plate-circuit Pt is utilized to apply a restraining Voltage to the grid-circuit G5 of a relay-tube RT, through a coupling-capacitor Cl B, a conductor 39, a voltage-doubler which is generically indicated at 46, and a grid-circuit resistor Rl2. The voltage-doubler, as explained in the Mehring patent, consists of a resistor RI 3, which is connected between the conductor 39 and the cathode-circuit 22 of the second gas tube V2, a capacitor CH, which is connected between the circuit 39 and a circuit 4i a doublecircuit rectifier-valve RV, and a loading-resistor Rl4.' The loading-resistor R-M is connected between the cathode-circuit 22 of the gas tube V2 and the anode circuit 42 of the right-hand rectifier of the rectifier-valve RV. The circuit 42 also constitutes one terminal of the grid-circuit resistor P,l2 of the receiver-tube RT. The cathode of the right-hand half of the doublerectifier valve RV is connected to the conductor- 4I, while the anode of the left-hand rectifiercircuit of the doubler valve RV is connected to the same conductor. The cathode of the lefthand rectifier of the double valve RV is connected, at 43, to the cathode-circuit 22 of the second gas valve V2. The loading-resistor Ri4 is by-passed by a Tadio-frequen0y by-pass capacitor BPC.

The relay-tube RT has its cathode-circuit 44 energized from a tapped point of potentiometer P05 which is connected between the conductor 15 and the positive supply-terminal The plate-circuit P5 of the relay-tube RT is connected through the primary winding of a relay outputtransformer ROT, a conductor 45, a make-contact 45 of the high-set fault-detector FD2, and thence to the positive terminal The screengrid circuit 8G5 of the relay-tube RT is also connected to the aforesaid conductor 45.

The relay output-transformer ROT has a secondary winding ll, which is utilized to energize the operating coil R of a relay R which has a single make-contact 48, which is shown, near the top of the negative supply-terminal as being in the tripping-circuit of the trip-coil TC of the circuit breaker 3, this trip-circuit extending from the negative bus to the positive bus and also including the auxiliary breaker-contact 3a.

In describing the operation of our invention, we will pay particular attention to the distinguishing features which distinguish the invention from the system which has already been described and claimed in the previously-mentioned Mehring Patent 2,408,868. When a fault of a predetermined severity occurs on the protected linesection I, one or both of the two fault-detectors FDI and FDZ will pick up. The low-set detector FDI is the more sensitive of the two, and when aaaasii it responds, it opens its back-contact 25, and thus removes a short-circuit from around the two gas tubes VI and V2. The grid-circuit settings of both of these gas-tubes VI and V2 are more sensitive than the sensitive fault-detector FDE, so that said tubes are already in readiness to fire, in response to the alternating voltages which are applied to their respective grids G5 and G2, as soon as the sensitive fault-detector FD! responds.

Whichever one of the two gas tubes Vi and V2 has a positive grid voltage applied to it, at the moment of opening of the FDE back-contact 25, will instantly begin firing, developing a certain positive voltage in its cathode-circuit M or 22, as the case may be, making said cathodecircuit positive with respect to the conductor 15. During the next half-cycle of the line-current, or output of the secondary winding 8 of the saturating transformer ST, a positive grid-voltage is applied to the other one of the two gas-tubes VI and V2, causing this other tube to fire, putting out the first-firing gas-tube, in the process, as explained in the Mehring patent. This process is called triggering the two gas-tubes Vi and V2. It will be understood, of course, that these two triggering tubes Vi and V2, since they are gastubes, are sustained-discharge tubes, that is, tubes in which the grid fires the tube, or starts the discharge, but is unable to extinguish the tube or interrupt the discharge, as explained in the Mehring patent. The two gas-tubes Vi and V2 thus operate as sources of two difierent series of fiat-topped voltage-waves of constant magnitude, one gas tube being responsive to positive line-frequency half-cycles, while the other is responsive to negative line-frequency half-cycles.

Carrier-current is transmitted by the firing of the master-oscillator tube 086 during the flattopped voltage impulses which are supplied from the cathode-circuit 2! of the first gas-tube Vi, thus transmitting a succession of bursts of can rier-current energy, which are applied to the line through the coupling capacitors CC, during line current half-cycles of one polarity. During the line-current half-cycles of the other polarity, positive or operating-voltage is applied to the grid-circuit G5 of the relay-tube RT through the cathode-circuit 22 of the second gas-tube V2, the voltage of the circuit 22 being applied to the gridcircuit G5 through the resistor Ft-i l, the conductor 42, and the resistor 3-42. This operating-voltage tends to cause the fiow of platecurrent in the relay-tube RT.

As explained in the l-Jlehring the carriercurrent impulses which are transmitted from both ends of the protected line-section are received in the receiver-tube REC at each end or" the line-section, but the only important received carrier-impulses, at either end, are those which are received from the far end of the protected line-section, because the carrier-current impulses, or bursts, or operating-periods, which are transmitted from the relaying station itself, always occur during the half-cycles of the line-current when no operating-voltage is applied to the gridcircuit G5 of the relay-tube RT from the cathodecircuit 22 of the second gas tube V2. end of the protected line-section, however, the carrier-current energy-impulses or bursts, which are transmitted at line-frequency hali-cycie intervals, are transmitted in response to the oathode-circuit 2| of the first gas tube V5 in that station, so that, if the line-current at the farend station is in phase with the line-current at the relaying station (as it will be, when there At the far is no fault in the protected line section), then the carrier-current transmitting-periods at the far end will exactly coincide with the operatingvoltage half-cycle periods at the relaying station, and thus they will block a response of the relaytube RT.

The received carrier-current energy is applied, in a blocking fashion, to the grid-circuit G5 of the relay-tube RT, through the coupling capacitor C-i $3 and the voltage-doubler ad, which operates as described in the Mehring patent, so as to build up a negative voltage, in the loading resistor R-Hi, which is at least as large as, and opposite in sign to, the positive grid-voltage which is supplied by the cathode-circuit 22 of the second gas tube V2.

The result of the foregoing operations is that the relay-tube RT will become conducting only when there is an internal fault, or a fault within the protected line-section i, in which case the tube will become conducting periodically, in short or long bursts, depending upon the phase-relations between the line currents at the opposite ends of the protected line-section. The alternating-current component of the plate-current of the relay-tube HT is applied to the operating coil of the phase-angle-responsive relay R, through the relay output-transformer HOT. The contact 58 of the relay R is then utilized to trip the breaker 3.

It will be noted that the carrier-current equipment acts as a pilot-channel connecting the two ends of the protected line-=section for the purpose of effecting a determination or comparison of the phase-angle between the two terminal linecurrents of the protected line-section.

In order to insure the proper operation of the relay R, in view of its dependence upon faultresponsive relay-operations at two widely-separated points, namely at the two opposite ends of the protected line-section, it is necessary to make sure that the restraining impulses which are applied to the relay-tube RT are applied at least as soon as, and usually a trifle ahead of, the effective application of the operating-impulses to the grid-circuit G5 of this relay tube RT.

In the Mehring system, this coordination was obtained by biasing the second gas-tube V2 so that it would have a pick-up value some 25% higher than the fault-current magnitude which resulted in a picking up of the single fault-detector relay which was utilized in said Mehring system, thereby entailing difficulties due to the impossibility of properly coordinating the gastube pick-up value with the mechanical-relay pick-up value, because of the dependence of the tube-operation on the station battery-voltage represented by the terminals and and also because of the much more sensitive responsiveness oi" the tube to short-term transients on both the alternating-current control-voltage of the tube, and the direct-current plate-voltage source for the tube. Even aside from these coordination problems, the Mehring system was subject to the possibility of a blind spot in its operation, in the event of a fault which was barely severe enough to pick up the mechanical faultdetector, but not severe enough to fire the second gas tube V2, and also in the case in which the fault-currents, which were fed into a fault from the opposite ends of the protected line-section, were unequal in value, so that the second gas tube at the end having the smaller current would not fire, resulting in a continuous or uninterrupted transmission of carrier, which would block a tripping-action at both ends of the protected line-section.

In accordance with our invention, we avoid these coordination-defects and blind-spot contingencies by having both of the gas-tubes Vi and V2 operative, or in condition for firing, at the lowest fault-current value which will result in a picking up of the low-set or sensitive faultdetector FD! Thus, when carrier is transmitted at all, it is always transmitted on alternate halfcycles of the line-current at the station where the carrier is being transmitted. In accordance with our invention, we secure the less-sensitive fault-detection coordination, by utilizing a second mechanical fault-detector relay FDZ, which responds to the same derived fault-current which is utilized to energize the first fault-detector relay FDI, but which has a slightly higher setting, such as responding to a current-value of approximately 125% of the pick up of the first fault-detector FDI.

In accordance with our invention, we also make use of a back-contact, 25 (rather than a makecontact), on the first fault-detector FDI, because a back-contact will not bounce, in response to a fault, thus avoiding the possibility of a momentary interruption of the transmission of a blocking or restraining signal at the beginning of a fault, thus resulting in false tripping at the opposite terminal. We also use a make-contact, 46, on the second fault-detector FD2, so that, even though both of the fault-detectors should pick up simultaneously, the detector having the back-contact will get that contact open before the other detector has moved far enough to close a normally open make-contact, thus insuring that a restraining-voltage is available, on the relay tube RT, a trifle ahead of the effective application of an operating-voltage to the relay tube RT.

It will be understood, of course, that the blocking of the operating voltage of the tube can be effected either by preventing the application of a sufficiently positive grid-voltage to the tube, or by preventing the application of a sufficient platevoltage to the tube. In the illustrated case, the blocking of the operating voltage is effected by preventing the application of a plate-voltage to the tube.

It will be noted, finally, that we avoid shockexcitation defects, due to responses to transients, by utilizing the same kind of mechanical relays for both of our fault-detectors FD! and FD2, so that our phase-angle relaying-system operates satisfactorily, without any of the disadvantages inherent in the Mehring system.

We claim as our invention:

1. Terminal equipment for one terminal of a pilot-channel phase-angle relaying-system for an alternating-current transmission-line, comprising the combination, with a relay to be controlled, and a pilot-channel means for communicating with another terminal of the protected line-section, of means for deriving an alternating-current line-current relaying quantity which is responsive to a plurality of different line-fault conditions, two line-current-responsive fault-detector means of differing sensitivities, line-currentresponsive impulse-producing means, operating under the supervision of the low-set fault-detector means, for applying a succession of operating-impulses, effective on said relay, in response to derived line-current half-cycles of one polarity, and for delivering a succession of pilot-channel-controlling impulses to said pilot-channel means in response to derived line-current impulses of the opposite polarity, receiving-means for applying restraining-impulses, effective on said relay, in response to pilot-channel impulses received from another terminal of the protected line-section, and means for supervising said relay in response to the high-set fault-detector means.

2. Terminal equipment for one terminal of a pilot-channel phase-angle relaying-system for an alternating-current transmission-line, comprising the combination, with a relay to be controlled, and a pilot-channel means for communicating with another terminal of the protected line-section, of two line-current-responsive fault-detector means of differing sensitivities, line-currentresponsive impulse-producing means, operating under the supervision of the low-set fault-detector means, for applying a succession of operating-impulses, effective on said relay, in response to derived line-current half-cycles of one polarity, and for delivering a succession of pilot-channel-controlling impulses to said pilot-channel means in response to derived line-current impulses of the opposite polarity, receiving-means for applying restraining-impulses, effective on said relay, in response to pilot-channel impulses received from another terminal of the protected line-section, and means for supervising said relay in response to the high-set fault-detector means, said operating-impulses, said pilot-channel-controlling impulses, and said restraining-impulses all being substantially flat-topped impulses of substantially constant magnitudes which are substantially independent of the fault-severity, the magnitude of the derived line-current, or the magnitude of the received energy which is received over said pilot channel.

3. The invention as defined in claim 1, characterized by the two fault-detector means being similarly operating, and similarly energized, equipments having different settings, both controlled in response to the same derived line-current which is used in said impulse-producing means.

4. The invention as defined in claim 1, characterized by the two fault-detector means being similarly operating, and similarly energized, mechanical relays having different settings, both controlled in response to the same derived linecurrent which is used in said impulse-producing means, the low-set fault-detector relay having a normally closed contact, which opens when the relay responds to a fault, and the high-set faultdetector relay having a normally open contact, which closes when the relay responds to a fault, the fault-detector supervision which is specified in claim 1 being effected by said contacts, respectively.

5. The invention as defined in claim 1, characterized by said impulse-producing means comprising two gas tubes of the sustained-discharge type, each tube having trigger-acting control-circuit means for firing the tube, direct-current plate-oathodecircuit energization-means for said tWo gas tubes, interconnecting impulsingmeans between the plate-cathode circuits of said two gas tubes for responding to the moment of firing of either tube in such manner as to so impulse the eifective plate-cathode voltage across the other tube as to extinguish said other tube, means for so applying said relaying quantity to the control-circuit means of the two gas tubes that the tubes fire during half-cycles of opposite polarity of said relaying quantity, the grid-circuit settings of both of said gas tubes being more sensitive than the sensitive fault-detector, means responsive to the discharge of one of said gas tubes for delivering a pilot-channel-controlling impulse to said pilot-channel means as long as said discharge lasts, and means responsive to the discharge of the other gas tube for applying an operating-impulse, effective on the relay, as long as said discharge lasts.-

HERBERT W. LENSNER. SHIRLEY L. GOLDSBORO-UGH. ROBERT C. CHEEK.

12 REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date Harder Jan. 17, 1939 Lenehan Jan. 17, 1939 Bostvvick Mar. 10, 1942 Lenehan Mar. 10, 1942 Bostwick et a1 Aug. 27, 1946 Mehring et al Oct. 8, 1946 

